Frequency offset estimator

ABSTRACT

A complex symbol sequence ( 13 ), which is a product of an orthogonally detected complex demodulated symbol sequence ( 1 ) and a conjugate complex number of a known symbol sequence ( 2 ), is applied to phase rotating units ( 4   a,    4   b,    4   c ) which change the phase of the complex symbol sequence ( 13 ) based on frequency offsets (fa, fb, fc). N-symbol adders ( 5   a,    5   b,    5   c ) each add N symbol values supplied from the phase rotating units ( 4   a,    4   b,    4   c ), while M-power adders ( 6   a,    6   b,    6   c ) each calculate the power of a value supplied from the N-symbol adders ( 5   a,    5   b,    5   c ), and add the power M times. A frequency offset control unit ( 7 ) controls frequency offsets applied to the phase rotating units ( 4   a,    4   b,    4   c ) based on three power sums supplied from the M-power adders ( 6   a,    6   b,    6   c ), and delivers a power offset estimate ( 8 ).

TECHNICAL FIELD

The present invention relates to a frequency offset estimator for use ina receiver which demodulates a signal modulated in accordance with anorthogonal modulation scheme.

BACKGROUND ART

The quasi-synchronous detection typically used as a detection method inlandline mobile communications requires that a transmission carrierfrequency matches a reference carrier frequency for quasi-synchronousdetection of a receiver. However, when oscillators in a transmitter anda receiver are not sufficiently high in frequency stability andaccuracy, this results in a difference in frequency between both sides.This is called a “frequency offset.” Since the frequency offset, if any,causes the phase of a detected signal to rotate, the signal cannot becorrectly demodulated. To prevent this incorrect demodulation, AFC(Automatic Frequency Control) is typically used. The AFC estimates afrequency offset in carrier frequency between the transmission side andreception side to control the frequencies of oscillators. The AFC alsocorrects demodulated signals for the rotated phase caused by thefrequency offset.

Conventionally, a frequency offset estimator for use in the AFC relieson the differential detection as can be seen in FIG. 2 in Laid-openJapanese Patent Application No. 8-213933, “Characteristics of ½ SymbolDifferential Detection AFC Having wide Frequency Pull-in Range,”Technical Report, RCS96-25, 1st-6th Paragraphs, June 1996, “FrequencyOffset Estimating Method in Fading Transmission Path with Large TimeDispersion,” Technical Report, RCS98-81, 13th-18th paragraphs, September1998, and the like. However, the differential detection type frequencyoffset estimator has a drawback that the accuracy of estimation issignificantly degraded when the carrier to noise power ratio (CNR) islow.

For example, a CDMA (Code Division Multiple Access) communication systemusing the quasi-synchronous detection effectively utilizes a pathdiversity effect resulting from rake combination. In addition, requiredSNR (Signal to Noise Power Ratio) may occasionally be on the order of 0dB at a bit error rate (BER) of 0.1%, resulting from the effects oferror correcting codes, transmission power control and the like. It istherefore necessary to provide an expedient which is capable ofestimating a frequency offset even at low CNR.

A conventional frequency offset estimator will be described withreference to FIG. 1. The differential detection frequency type offsetestimator shown in FIG. 1 estimates a frequency offset from a differencein phase between symbols. First, complex multiplier 3 calculates aproduct of orthogonally detected complex demodulated symbol sequence 1and a complex conjugate of a known symbol sequence 2 correspondingthereto. The product is fed to differential detector 15 as complexsymbol sequence 13. Differential detector 15 delays complex symbolsequence 13 by several symbols using delay circuit 16, and complexmultiplier 3 calculates a product of a complex conjugate of the delayedsymbol sequence, which have passed through delay circuit 16, andoriginal complex symbol sequence 13. This product is provided toaveraging circuit 17 for averaging, and then delivered as frequencyoffset estimate 18. In this event, as larger noise is added toorthogonally detected complex demodulated symbol sequence 1, largervariations occur in the phase difference between symbols, resulting in adegraded accuracy of estimation for frequency offset. It is known thatthe accuracy of estimation is improved to some extent even using thedifferential detection, if the number of delayed symbols is increased.This is because the phase difference between symbols becomes largerrelative to variations in phase due to noise. Disadvantageously,however, an increased number of delayed symbols results in a narrowerrange in which a frequency offset can be estimated. Therefore, when thedifferential detection type frequency offset estimator is used, atradeoff is inevitably made between the accuracy of estimation and anestimatable range in regard to the number of delayed symbols.

On the other hand, there has been proposed an FFT (Fast FourierTransform) based method as another frequency offset estimating method.This type of estimating method converts received symbols into afrequency domain by FFT, and determines a frequency indicative of a peakof a spectrum envelope as a frequency offset. This estimating methodprovides a higher accuracy of estimation at low CNR than thedifferential detection since the peak can be relatively easily foundeven if a received signal presents low CNR. However, the accuracy ofestimation depends on the order of FFT. An article “FFT-Based HighlyAccurate Frequency Determining Method,” Transactions-A of the Instituteof Electronics, Information and Communication Engineers, Vol. J 70-A,No. 5, pp.798–805 describes that FFT should be used at 32 points or morefor estimating a frequency using FFT. However, the FFT cannot be used at32 points or more in some occasions.

Conventionally, known symbols transmitted in a predetermined order havebeen used for estimating a frequency offset. In mobile communications, asection comprised of several symbols is called a “slot” which containspilot symbols, data symbols, control symbols and the like. The pilotsymbols refer to known symbols which are transmitted in a predeterminedorder. While the total number of symbols in a slot ranges from aboutfifteen to several hundreds, the number of pilot symbols is generallysmaller than the number of data symbols.

Taking as an example, the international standard IMT-2000 for the nextgeneration mobile communications, as described in an article “3G TS25.211 version 3.2.0, 3rd Generation Partnership Project: TechnicalSpecification Group Radio Access Network; Physical channels and mappingof transport channels onto physical channels (FDD),” one slot includesonly 16 pilot symbols at most even at a high bit rate. In other words,when a frequency offset is estimated using sequential pilot symbols inone slot interval, a sufficient number of pilot symbols is not providedfor utilizing the FFT. When pilot symbols are used over a plurality ofslot intervals, a frequency offset can be estimated in a narrower range.

On the other hand, a peak detection type frequency offset estimatingmethod has also been proposed, as can be seen in FIG. 1 in Laid-openJapanese Patent Application No. 9-200081. The proposed frequency offsetestimating method, which is for use in a direct code spreadcommunication system, involves despreading baseband complex signalsorthogonally detected using complex spread codes previously applied withfrequency offsets which have an equal absolute value and differentsigns, averaging several symbols acquired at timings at which a maximalpeak is detected, and converting the power value of the average to afrequency offset using a previously measured conversion table. Thisestimating method is considered to provide a better accuracy ofestimation at low CNR than the differential detection type since it usesan average value of symbols which are despread by spreading codesapplied with frequency offsets. However, a conversion table must bepreviously created for calculating a frequency offset. When theconversion table is used, it can be thought that if the characteristicsof devices vary from one apparatus to another, a resulting frequencyoffset may be different depending on a particular apparatus, so that thecreation of a conversion table, in general, is not an easy task. Inaddition, a memory is required for storing the conversion table.Moreover, a correction may be required for suppressing variations in thecharacteristics of devices between apparatuses.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a frequency offsetestimator which is less susceptible to a degraded accuracy of estimationeven at low CNR in a CDMA communication system.

According to a first aspect of the present invention, the frequencyoffset estimator comprises complex multiplying means and frequencyoffset estimating means. The frequency offset estimating means includesa plurality of power sum calculating means and frequency offset controlmeans.

The complex multiplying means receives an orthogonally detected complexdemodulated symbol sequence, and calculates a product of the complexdemodulated symbol sequence and a conjugate complex number of a knownsymbol sequence corresponding to the complex demodulated symbol sequenceto remove a symbol information component.

Each of the power sum calculating means calculates a power sum for thecomplex demodulated symbol sequence based on an applied frequency offsetafter the symbol information component has been removed, and includesphase rotating means for changing the phase of the complex demodulatedsymbol sequence based on the frequency offset applied thereto after thesymbol information component has been removed, N-symbol adding means foradding a plurality of complex symbols delivered from the phase rotatingmeans, and M-power adding means for calculating the power of the complexsymbol sum calculated by the N-symbol adding means, and adding the powerof a plurality of symbols.

The frequency offset control means controls a frequency offset appliedto the power sum calculating means based on the power sums calculated bythe plurality of power sum calculating means, estimates a frequencyoffset included in the complex demodulated symbol sequence, and deliversthe estimated frequency offset.

In this manner, the utilization of a gain resulting from the in-phaseaddition is utilized in a frequency offset estimation, and averaging ofthe in-phase added power values through the addition of power, resultsin the ability to estimate a frequency offset at CNR higher by severaldB than CNR of received carrier power.

According to a second aspect of the present invention, a frequencyoffset estimator comprises a plurality of complex multiplying means,maximal-ratio combining means, a plurality of power sum calculatingmeans, and frequency offset control means.

Each complex multiplying means receives an orthogonally detected complexdemodulated symbol sequence, and calculates a product of the complexdemodulated symbol sequence and a conjugate complex number of a knownsymbol sequence corresponding to the complex demodulated symbol sequenceto remove a symbol information component included in each complexdemodulated symbol sequence. The maximal-ratio combining means combinesa plurality of complex demodulated symbol sequences, from which symbolinformation component has been removed, at a maximal-ratio to generate asingle complex symbol sequence. The power sum calculating means andfrequency offset control means are identical to the counterparts in thefirst aspect.

According to a third aspect of the present invention, a frequency offsetestimator comprises a plurality of the frequency offset estimators inthe first aspect, and a maximal-ratio combiner for combining frequencyoffset estimates of these frequency offset estimators at a maximalratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a conventional frequency offsetestimator based on the differential detection;

FIG. 2 is a block diagram illustrating a frequency offset estimatoraccording to a first embodiment of the present invention;

FIG. 3 is a diagram showing the relationship between the power spectrumof a signal including a frequency offset fz and frequency offsets fa,fb, fc controlled by frequency offset control unit 7;

FIG. 4 is a block diagram illustrating a specific example of frequencyoffset control unit 7;

FIG. 5 is a block diagram illustrating a frequency offset estimatoraccording to a second embodiment of the present invention; and

FIG. 6 is a block diagram illustrating a frequency offset estimatoraccording to a third embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

Referring to FIG. 2, a frequency offset estimator according to a firstembodiment of the present invention uses an orthogonally detectedcomplex demodulated symbol sequence 1 and a known symbol sequence 2corresponding thereto as input signals, and comprises complex multiplier3 and frequency offset estimating unit 11. Frequency offset estimatingunit 11 comprises power sum calculating units 20 a, 20 b, 20 c, andfrequency offset control unit 7. Power sum calculating unit 20 acomprises phase rotating unit 4 a, N-symbol adder 5 a, and M-power adder6 a. Similarly, power sum calculating units 20 b, 20 c comprise phaserotating units 4 b, 4 c; N-symbol adders 5 b, 5 c; and M-power adders 6b, 6 c, respectively.

First, complex multiplier 3 calculates a product of orthogonallydetected complex demodulated symbol sequence 1 and a conjugate complexnumber of known symbol sequence 2. This processing can remove a symbolinformation component included in the demodulated symbol sequence toextract complex symbol sequence 13, the phase of which is rotated by afrequency offset. Used as known symbol sequence 2 may be pilot symbolswhich are generally used in a communication system based on theorthogonal modulation. Alternatively, in a direct code spreadcommunication system, products of known spreading codes and pilotsymbols can be used as known symbol sequence 2. Conventionally, afrequency offset has been estimated by the delayed detection using thiscomplex symbol sequence 13.

In this embodiment, three different frequency offsets are applied tocomplex symbol sequence 13, a plurality of resulting symbols are added,and the power is added over the plurality of symbols. The frequencyoffsets applied to complex symbol sequence 13 are appropriatelycontrolled based on three power sums thus calculated. This processing isrepeated several times to estimate a frequency offset.

Complex symbol sequence 13 delivered from complex multiplier 3 isprovided to phase rotating units 4 a, 4 b, 4 c. Phase rotating units 4a, 4 b, 4 c change the phase of complex symbol sequence 13 based onfrequency offsets fa, fb, fc, respectively, applied thereto fromfrequency offset control unit 7. Frequency offsets fa, fb, fc areapplied as the amount of phase change per symbol. Phase rotating unit 4a rotates the phase of complex symbol sequence 13 by frequency offset fain the negative direction. Similarly, phase rotating units 4 b, 4 crotate the phase of complex symbol sequence 13 by frequency offset fb,fc, respectively, in the negative direction. Complex symbols deliveredfrom phase rotating unit 4 a are provided to N-symbol adder 5 a; thosefrom phase rotating unit 4 b to N-symbol adder 5 b; and those from phaserotating unit 4 c to N-symbol adder 5 c, respectively. N-symbol adders 5a, 5 b, 5 c each add N symbols of values provided from phase rotatingunits 4 a, 4 b, 4 c, and N-symbol adder 5 a supplies the resulting valueafter the addition to M-power adder 6 a; N-symbol adder 5 b to M-poweradder 6 b; and N-symbol adder 5 c to M-power adder 6 c, respectively,where N is an integer equal to or larger than two.

If any of frequency offsets fa, fb, fc supplied from frequency offsetcontrol unit 7 is close to a true frequency offset to be estimated, thesum of symbols applied with that frequency offset takes a larger valuethan the sums of symbols applied with other frequency offsets. Further,the additions made in N-symbol adders 5 a, 5 b, 5 c relatively reducethe proportion of additive white Gauss noise which accounts for symbolsafter the additions.

M-power adders 6 a, 6 b, 6 c respectively calculate the power of valuessupplied from N-symbol adders 5 a, 5 b, 5 c, and repeat the addition ofthe power M times where M is an integer equal to or larger than two. Inthis manner, the power values of the sums delivered from N-symbol adders5 a, 5 b, 5 c are averaged. The three power sums thus calculated areprovided to frequency offset control unit 7. N-symbol adders 5 a, 5 b, 5c add N symbols, and M-power adders 6 a, 6 b, 6 c add the power M times,so that M-power adders 6 a, 6 b, 6 c each supply one power sum tofrequency offset control unit 7 using N×M complex demodulated symbolswhich are orthogonally detected.

Frequency offset control unit 7 controls frequency offsets applied tophase rotating units 4 a, 4 b, 4 c based on the three power sumssupplied from M-power adders 6 a, 6 b, 6 c. The three power valuescalculated by adding complex symbols, while applying the frequencyoffsets thereto, and adding the power, are larger as the frequencyoffset applied thereto are closer to a true frequency offset to beestimated. Thus, based on the magnitude relationship, frequency offsetcontrol unit 7 can control frequency offsets fa, fb, fc applied to phaserotating units 4 a, 4 b, 4 c.

As frequency offset control unit 7 updates frequency offsets fa, fb, fcapplied to phase rotating units 4 a, 4 b, 4 c, the power sums arecalculated again by phase rotating units 4 a, 4 b, 4 c, N-symbol adders5 a, 5 b, 5 c, and M-power adders 6 a, 6 b, 6 c using the updatedfrequency offsets. Frequency offset control unit 7 appropriatelycontrols frequency offsets fa, fb, fc again using the calculated powersums. After repeating the calculation of power sums and the update offrequency offsets several times, frequency offset control unit 7 selectsone from frequency offsets fa, fb, fc for delivery as frequency offsetestimate 8.

Next, the magnitude relationship among the three power sums calculatedby M-power adders 6 a, 6 b, 6 c will be described with reference toFIGS. 3 a, 3 b. FIGS. 3 a, 3 b show a power spectrum distribution ofcomplex symbol sequence 13 in FIG. 2, where the horizontal axisrepresents the frequency, and the vertical axis represents the magnitudeof power spectrum. Since complex symbol sequence 13 in FIG. 2 is theproduct of orthogonally detected complex demodulated symbol sequence 1and a complex conjugate of known symbol sequence 2, a peak should existin a direct current component if no frequency offset is present. Whenpositive frequency offset fz is included, the power spectrum of complexsymbol sequence 13 presents a simple convex waveform with the centerfrequency being shifted by fz in the positive direction, as shown inFIGS. 3 a, 3 b. In other words, the power spectrum is largest atfrequency offset fz, and is reduced as it is further away from fz.

Assume now that in the frequency offset estimator of FIG. 2, frequencyoffsets applied to phase rotating units 4 a, 4 b, 4 c are designated fa,fb, fc, respectively, and are placed in a magnitude relationshiprepresented by fa<fb<fc. Further, when the power spectra at respectivefrequency offsets fa, fb, fc are designated Pa, Pb, Pc, respectively,the magnitude relationship among Pa, Pb, Pc varies depending on themagnitude relationship among fa, fb, fc and fz. As shown in FIG. 3 a,when fa<fb<fc<fz, the magnitude relationship among the power spectra isrepresented by Pa<Pb<Pc. Conversely, when fz<fa<fb<fc, Pa>Pb>Pc isestablished as shown in FIG. 3 b. Frequency offset control unit 7 relieson the foregoing relationships to control the frequency offsets appliedto phase rotating units 4 a, 4 b, 4 c.

Next, a specific example of frequency offset control unit 7 will besupplied with reference to FIG. 4. Frequency offset control unit 7 isapplied with power values Pa, Pb, Pc from M-power adders 6 a, 6 b, 6 c,respectively. Pa is a power sum calculated by N-symbol adder 5 a andM-power adder 6 a while frequency offset fa is applied to phase rotatingunit 4 a. Pb is a power sum calculated by N-symbol adder 5 b and M-poweradder 6 b while frequency offset fb is applied to phase rotating unit 4b. Pc is a power sum calculated by N-symbol adder 5 c and M-power adder6 c while frequency offset fc is applied to phase rotating unit 4 c.Frequency offset control unit 7 updates frequency offsets fa, fb, fcapplied to phase rotating units 4 a, 4 b, 4 c using these three powersums Pa, Pb, Pc. Frequency offset control unit 7 utilizes theaforementioned relationships among fz, fa, fb, fc and power spectra, andcompares power sums Pa, Pb, Pc with one another to determine fcand. Anexemplary process for determining fcand is shown below.

(1) When power sums Pa, Pc, Pc are in a magnitude relationshiprepresented by Pa>Pb>Pc, fcand=fa.

(2) When power sums Pa, Pc, Pc are in a magnitude relationshiprepresented by Pc>Pb>Pa, fcand=fc.

(3) When power sums Pa, Pb, Pc are in a magnitude relationship whichdoes not fall under either (1) or (2), fcand=fb.

Furthermore, a value Δf is used. One-half of current Δf value is used asthe next Δf:Δf=Δf/2

Frequency offset control unit 7 updates frequency offsets fa, fb, fc inthe following manner using fcand and Δf thus determined:fa=fcand+Δffb=fcandfc=fcand−Δf

In accordance with the foregoing method of determining fa, fb, fc, anyof the values fa, fb, fc before updating are substituted into fb.Therefore, a power sum corresponding to fcand of power sums Pa, Pb, Pccalculated before the update can be used as Pb as it is when fa, fb, fcare updated the next time. In other words, when frequency offset controlunit 7 first updates fa, fb, fc, phase rotating units 4 a, 4 b, 4 c,N-symbol adders 5 a, 5 b, 5 c, and M-power adders 6 a, 6 b, 6 c arefully operated to calculate power sums Pa, Pb, Pc. However, once fa, fb,fc are updated, it is not necessary to operate phase rotating unit 4 b,N-symbol adder 5 b, and M-power adder 6 b. In the foregoing manner,frequency offset control unit 7 delivers fcand as frequency offsetestimate 8 after it has updated frequency offsets fa, fb, fc severaltimes. Of course, frequency offset control unit 7 may employ a method ofdetermining fa, fb, fc other than that described above.

Second Embodiment

Next, a second embodiment of the present invention will be describedwith reference to FIG. 5. A frequency offset estimator according to thesecond embodiment uses P orthogonally detected complex demodulatedsymbol sequences 1 (#1–#P), and known symbol sequences 2 (#1–#P)corresponding thereto as input signals, and estimates a frequency offsetusing complex multiplier 3, maximal-ratio combiner 9, and frequencyoffset estimating unit 11. Frequency offset estimating unit 11 hascompletely the same functions as frequency offset estimating unit 11 inFIG. 2.

P complex demodulated symbol sequences 1 may be, for example, signalsfrom different antennas, or multipath signals, for example, in a directspread communication system. The frequency offset estimator can expect acertain gain and also provide redundancy by the use of a plurality ofcomplex demodulated symbol sequences 1.

Complex multiplier 3 calculates products of orthogonally detectedcomplex demodulated symbol sequences #1–190 P and conjugate complexnumbers of known symbol sequences #1–190 P corresponding thereto, andsupplies the products to maximal-ratio combiner 9. Maximal-ratiocombiner 9 first estimates CNRs of complex symbol sequences #1–190 Psupplied from complex multiplier 3. Next, Maximal-ratio combiner 9determines a weighting coefficient for each symbol sequence from CNR ofeach symbol sequence so as to provide maximal CNR after combination.Then, after weighting respective symbol sequences #1–190 P using thedetermined weighting coefficients, the resulting symbol sequences arecombined in phase, and delivered as complex symbol sequence 14. Complexsymbol sequence 14, generated by combining complex symbol sequences#1–190 P at the maximal CNR, is provided to frequency offset estimatingunit 11. The frequency offset estimating unit 11 operates completely inthe same manner as that shown in FIG. 2.

Third Embodiment

Next, a third embodiment of the present invention will be described withreference to FIG. 6. A frequency offset estimator according to the thirdembodiment uses P orthogonally detected complex demodulated symbolsequences 1 (#1–190 P), and known symbol sequences 2 (#1–190 P)corresponding thereto, and comprises P frequency offset estimating units12 in the first embodiment, each for independently estimating afrequency offset for each complex demodulated symbol sequence 1; andmaximal-ratio combiner 19 for combining P frequency offset estimatesestimated by these frequency offset estimating units 12 at a maximalratio.

First, orthogonally detected complex demodulated symbol sequences #1–190P are supplied to P frequency offset estimating units 12. Frequencyoffset estimating units 12 are identical to the frequency offsetestimating unit in the first embodiment. Frequency offset estimatingunits 12 each estimate frequency offset estimate 8 using correspondingcomplex symbol sequences #1–190 P. Maximal-ratio combiner 19 firstestimates CNR of complex symbol sequence 13 after information symbolcomponents are removed from complex symbol sequences #1–190 P. Next,maximal-ratio combiner 19 determines a weighting coefficient for eachsymbol sequence from the CNR of each symbol sequence to provide maximalCNR when it combines complex symbol sequences 13 for complex symbolsequences #1–190 P. Then, maximal-ratio combiner 19 adds, withweighting, P frequency offset estimates 8 corresponding to complexdemodulated symbol sequences #1–190 P using the determined weightingcoefficients, and delivers the resulting sum as frequency offsetestimate 10.

1. A frequency offset estimator comprising: complex multiplying meansfor receiving an orthogonally detected complex demodulated symbolsequence, and calculating a product of said complex demodulated symbolsequence and a conjugate complex number of a known symbol sequencecorresponding to said complex demodulated symbol sequence to remove asymbol information component; a plurality of power sum calculating meanseach for calculating a power sum for said complex demodulated symbolsequence based on an applied frequency offset after the symbolinformation component has been removed; and frequency offset controlmeans for controlling a frequency offset applied to said each power sumcalculating means based on power sums calculated by said plurality ofpower sum calculating means, estimating a frequency offset included insaid complex demodulated symbol sequence, and delivering the estimatedfrequency offset, wherein said each power sum calculating means includephase rotating means for changing the phase of the complex demodulatedsymbol sequence based on the frequency offset applied thereto after thesymbol information component has been removed, N-symbol adding means foradding a plurality of complex symbols delivered from said phase rotatingmeans, and M-power adding means for calculating the power of the complexsymbol sum calculated by said N-symbol adding mean while adding thepower of a plurality of symbols.
 2. A frequency offset estimatorcomprising: a plurality of complex multiplying means each for receivingone of a plurality of orthogonally detected complex demodulated symbolsequences, and calculating a product of each said complex demodulatedsymbol sequence and a conjugate complex number of a known symbolsequence corresponding to each said complex demodulated symbol sequenceto remove a symbol information component included in said complexmodulated symbol sequence; maximal-ratio combining means for combiningsaid plurality of complex demodulated symbol sequences, from which thesymbol information components are removed, at a maximal ratio togenerate a single complex symbol sequence; a plurality of power sumcalculating means for calculating a power sum for said complex symbolsequence after the maximal-ratio combination based on applied frequencyoffsets; and frequency offset control means for controlling a frequencyoffset applied to said each power sum calculating means based on powersums calculated by said plurality of power sum calculating means,estimating a frequency offset included in said complex demodulatedsymbol sequence, and delivering the estimated frequency offset, whereinsaid each power sum calculating mean include phase rotating means forchanging the phase of said complex symbol sequence after themaximal-ratio combination based on the applied frequency offset,N-symbol adding means for adding a plurality of complex symbolsdelivered by said phase rotating means, and M-power adding means forcalculating the power of a complex symbol sum calculated by saidN-symbol adding means, and adding the power of a plurality of symbols.3. A frequency offset estimator comprising: a plurality of frequencyoffset estimating units each including complex multiplying means forreceiving an orthogonally detected complex demodulated symbol sequence,and calculating a product of said complex demodulated symbol sequenceand a conjugate complex number of a known symbol sequence correspondingto said complex demodulated symbol sequence to remove a symbolinformation component; a plurality of power sum calculating means eachfor calculating a power sum for said complex demodulated symbol sequencebased on an applied frequency offset after the symbol informationcomponent has been removed; and frequency offset control means forcontrolling a frequency offset applied to said each power sumcalculating means based on power sums calculated by said plurality ofpower sum calculating means, estimating a frequency offset included insaid complex demodulated symbol sequence, and delivering the estimatedfrequency offset, wherein said each power sum calculating means includephase rotating means for changing the phase of the complex demodulatedsymbol sequence based on the frequency offset applied thereto after thesymbol information component has been removed, N-symbol adding means foradding a plurality of complex symbols delivered from said phase rotatingmeans, and M-power adding means for calculating the power of the complexsymbol sum calculated by said N-symbol adding means, and adding thepower of a plurality of symbols; and a maximal-ratio combiner forcombining frequency offset estimates provided by said plurality offrequency offset estimating units at a maximal ratio to generate asingle frequency offset estimate.
 4. A frequency offset estimatorcomprising: symbol information removing means for receiving anorthogonally detected complex demodulated symbol sequence, and removinga symbol information component; a plurality of power sum calculatingmeans each for calculating a power sum for said complex demodulatedsymbol sequence based on an applied frequency offset after the symbolinformation component has been removed; and frequency offset controlmeans for controlling a frequency offset applied to said each power sumcalculating means based on power sums calculated by said plurality ofpower sum calculating means, estimating a frequency offset included insaid complex demodulated symbol sequence, and delivering the estimatedfrequency offset, wherein said each power sum calculating means include:phase rotating means for changing the phase of the complex demodulatedsymbol sequence based on the frequency offset applied thereto after thesymbol information component has been removed, N-symbol adding means foradding one or more complex symbols delivered from said phase rotatingmeans, and M-power adding means for calculating the power of the complexsymbol sum calculated by said N-symbol adding mean while adding thepower of one or more symbols.
 5. A frequency offset estimatorcomprising: a plurality of symbol information removing means each forreceiving one of a plurality of orthogonally detected complexdemodulated symbol sequences, and removing a symbol informationcomponent included in said complex modulated symbol sequence;maximal-ratio combining means for combining said plurality of complexdemodulated symbol sequences, from which the symbol informationcomponents are removed, at a maximal ratio to generate a single complexsymbol sequence; a plurality of power sum calculating means forcalculating a power sum for said complex symbol sequence after themaximal-ratio combination based on applied frequency offsets; andfrequency offset control means for controlling a frequency offsetapplied to said each power sum calculating means based on power sumscalculated by said plurality of power sum calculating means, estimatinga frequency offset included in said complex demodulated symbol sequence,and delivering the estimated frequency offset, wherein said each powersum calculating mean include: phase rotating means for changing thephase of said complex symbol sequence after the maximal-ratiocombination based on the applied frequency offset, N-symbol adding meansfor adding one or more complex symbols delivered by said phase rotatingmeans, and M-power adding means for calculating the power of a complexsymbol sum calculated by said N-symbol adding means, and adding thepower of one or more symbols.
 6. A frequency offset estimatorcomprising: a plurality of frequency offset estimating units eachincluding: symbol information removing means for receiving anorthogonally detected complex demodulated symbol sequence, and removinga symbol information component, a plurality of power sum calculatingmeans each for calculating a power sum for said complex demodulatedsymbol sequence based on an applied frequency offset after the symbolinformation component has been removed, and frequency offset controlmeans for controlling a frequency offset applied to said each power sumcalculating means based on power sums calculated by said plurality ofpower sum calculating means, estimating a frequency offset included insaid complex demodulated symbol sequence, and delivering the estimatedfrequency offset, wherein each of said plurality of power sumcalculating means include: phase rotating means for changing the phaseof the complex demodulated symbol sequence based on the frequency offsetapplied thereto after the symbol information component has been removed,N-symbol adding means for adding one or more complex symbols deliveredfrom said phase rotating means, and M-power adding means for calculatingthe power of the complex symbol sum calculated by said N-symbol addingmeans, and adding the power of said one or more a symbols; and amaximal-ratio combiner for combining frequency offset estimates providedby said plurality of frequency offset estimating units at a maximalratio to generate a single frequency offset estimate.
 7. A frequencyoffset estimating method comprising: a) receiving an orthogonallydetected complex demodulated symbol sequence, and removing a symbolinformation component; b) calculating a power sum for said complexdemodulated symbol sequence based on an applied frequency offset afterthe symbol information component has been removed; and c) controlling afrequency offset applied to said step b) based on power sums calculatedat said step b), estimating a frequency offset included in said complexdemodulated symbol sequence, and delivering the estimated frequencyoffset, wherein said step b) further includes a phase rotating step forchanging the phase of the complex demodulated symbol sequence based onthe frequency offset applied thereto after the symbol informationcomponent has been removed, an N-symbol adding step for adding one ormore complex symbols delivered from said phase rotating step, and anM-power adding step for calculating the power of the complex symbol sumcalculated at said N-symbol adding step while adding the power of one ormore symbols.
 8. A frequency offset estimating method comprising: a)receiving one of a plurality of orthogonally detected complexdemodulated symbol sequences, and removing a symbol informationcomponent included in said complex modulated symbol sequence; b)combining said plurality of complex demodulated symbol sequences, fromwhich the symbol information components are removed, at a maximal ratioto generate a single complex symbol sequence; c) calculating a power sumfor said complex symbol sequence after the maximal-ratio combinationbased on applied frequency offsets; and d) controlling a frequencyoffset applied to said step b) based on power sums calculated at saidstep b), estimating a frequency offset included in said complexdemodulated symbol sequence, and delivering the estimated frequencyoffset, wherein said step c) further includes: a phase rotating step forchanging the phase of said complex symbol sequence after themaximal-ratio combination based on the applied frequency offset, anN-symbol adding step for adding one or more complex symbols delivered atsaid phase rotating step, and an M-power adding step for calculating thepower of a complex symbol sum calculated at said N-symbol adding step,and adding the power of said one or more symbols.